Solar cells may be defined as devices to convert light energy into electrical energy by using a photovoltaic effect of generating electrons when light is incident onto a P—N junction diode. The solar cell may be classified into a silicon solar cell, a compound semiconductor solar cell mainly including a group I-III-VI compound or a group III-V compound, a dye-sensitized solar cell, and an organic solar cell according to materials constituting the junction diode.
A solar cell made from CIGS (CuInGaSe), which is one of group I-III-VI Chal-copyrite-based compound semiconductors, represents superior light absorption, higher photoelectric conversion efficiency with a thin thickness, and superior electro-optic stability, so the CIGS solar cell is spotlighted as a substitute for a conventional silicon solar cell.
In general, a CIGS solar cell can be prepared by sequentially forming a back electrode layer, a light absorbing layer, a buffer layer and a front electrode layer on a glass substrate. The substrate can be prepared by using various materials, such as soda lime glass, stainless steel and polyimide (PI). The back electrode layer mainly includes molybdenum (Mo) having low specific resistance and thermal expansion coefficient similar to that of the glass substrate.
The light absorbing layer is a P type semiconductor layer and mainly includes CuInSe2 or Cu(InxGa1−x)Se2, which is obtained by replacing a part of In with Ga. The light absorbing layer can be formed through various processes, such as an evaporation process, a sputtering process, a selenization process or an electroplating process.
The buffer layer is disposed between the light absorbing layer and the front electrode layer, which represent great difference in lattice coefficient and energy bandgap, to form a superior junction therebetween. The buffer layer mainly includes cadmium sulfide prepared through chemical bath deposition (CBD).
The front electrode layer is an N type semiconductor layer and forms a PN junction with respect to the light absorbing layer together with the buffer layer. In addition, since the front electrode layer serves as a transparent electrode at a front surface of the solar cell, the front electrode layer mainly includes aluminum-doped zinc oxide (AZO) having the superior light transmittance and electric conductivity.
In general, the light absorbing layer selectively absorbs light having the wavelength of about 500 nm to about 1200 nm. In particular, the light having the long wavelength exceeding 1200 nm may transmit through the light absorbing layer without being absorbed in the light absorbing layer. Thus, the CIGS solar cell according to the related art may use only the light having the wavelength of about 500 nm to about 1200 nm.